The present application relates to a battery including an electrolyte containing an electrolytic solution and a polymer compound.
In recent years, portable electronic devices such as a combination camera (video tape recorder), a mobile phone, and a notebook personal computer have been widely used. Downsizing, weight saving, and long life of such devices have been strongly demanded. Accordingly, as a power source of the portable electronic devices, a battery, in particular a light weight secondary battery that can provide a high energy density and a high output density has been developed.
A battery includes a cathode, an anode, and an electrolyte working as ion conduction path between the electrodes inside a metal can. In the battery, when the distance between the cathode and the anode is sufficiently large, the internal short circuit is hardly generated. However, the volume of the electrolyte that does not actually distribute to the battery life is designed small as much as possible. Thus, there is a tendency that the internal short circuit between the electrodes is easily generated. Therefore, to prevent internal short circuit even when the distance between the cathode and the anode is designed small, a separator made of a film, a nonwoven fabric, or paper is inserted between the electrodes.
As an electrolyte, a liquid electrolyte is widely used. Such a kind of electrolyte is generally called “electrolytic solution,” and contains a solvent and an electrolyte salt. As the electrolytic solution, in addition to an inorganic electrolyte such as sulfuric acid aqueous solution for a lead battery and potassium hydroxide aqueous solution for a dry cell, an organic electrolyte such as propylene carbonate solution and the like are known. The electrolytic solution is an ionic liquid with the high salt concentration. An electrolytic solution may show the strong acidity or strong alkalinity, or may be a nonaqueous solution according to the specification and the purpose of the battery. Therefore, in the battery including the electrolytic solution, when the electrolytic solution is leaked, the electric circuit or the like may be corroded, or resin parts may be dissolved. Thus, to stably use the battery, it is important to prevent leakage of the electrolytic solution.
Accordingly, a method for gelating an electrolytic solution by using a polymer compound has been suggested (for example, refer to Japanese Unexamined Patent Application Publication No. 2001-167797). Such a kind of electrolyte is generally called “gel electrolyte.” In the gel electrolyte, the electrolytic solution is held by a polymer compound. Therefore, the electrolytic solution is prevented from being leaked. In addition, the gel electrolyte closely contacts with and supports the cathode, the anode, and the separator and the like, and thereby the cathode, the anode, and the separator and the like have the self bearing properties. Thus, as a package member of the battery, a film package member can be used instead of a metal package member. In this case, the package member becomes light-weight, and thus the battery itself becomes light-weight. Further, complicated parts for structuring the battery are not necessitated. In the result, the battery can be manufactured at a lower cost. Furthermore, the film package member has the superior workability, and thus the degree of freedom of the battery shape is widened. Consequently, the battery including the gel electrolyte is superior not only in terms of preventing leakage of the electrolytic solution, but also in terms of attaining a light-weight battery, low manufacturing cost, and expansion of degree of freedom of the shape.
For the polymer compound used for the gel electrolyte, for the purpose of improving various performances, several polymers have been already suggested. More specifically, to favorably secure the electric connection between the cathode/the anode and the gel electrolyte, it is known that a binary system copolymer containing vinylidene fluoride and hexafluoropropylene as a component is used (for example, refer to Japanese Unexamined Patent Application Publication No. 11-312536). Further, to improve the load characteristics, the low temperature characteristics, and the cycle characteristics, it is known that a ternary system copolymer containing vinylidene fluoride, hexafluoropropylene, and monochlorotrifluoroethylene as a component, or a ternary system copolymer containing vinylidene fluoride, hexafluoropropylene, and monomethylmaleic acid ester as a component is used (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2006-114254 and 2002-008723).
In addition that the polymer is used for the gel electrolyte, it has been suggested that the polymer is used for a cathode binder and an anode binder. More specifically, it is known that polyvinylidene fluoride is used (for example refer to Japanese Unexamined Patent Application Publication Nos. 2001-273895 and 2002-110249); a binary system copolymer containing vinylidene fluoride and monomethylmaleic acid ester as a component is used (for example, refer to International Publication No. WO2004/049475 and Japanese Unexamined Patent Application Publication No. 2004-55493); and a binary system copolymer containing vinylidene fluoride and chlorotrifluoroethylene as a component is used (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-087325).
However, to improve the performance of the battery including the gel electrolyte, the gel electrolyte should have the sufficient liquid retaining characteristics. However, when the salt concentration of the electrolytic solution is increased to improve the battery capacity, the interaction between the solvent and the electrolyte salt becomes stronger. In the result, the interaction between the solvent and the polymer compound becomes weaker relatively, resulting in lowering of the liquid retaining characteristics of the gel electrolyte. In this case, when the electrolytic solution is released from the polymer compound, the contact characteristics between the gel electrolyte and the cathode/the anode/the separator becomes lowered. In the result, the battery performance such as the battery capacity, the cycle characteristics, and the load characteristics is lowered. Furthermore, if a defect such as a break in the film package member exists, the electrolytic solution is leaked in spite of using the gel electrolyte, resulting in lowering of the battery performance as well.